Correct development of Primordial Germ Cells (PGCs), the embryonic precursors of the gametes, is a prerequisite for adult fertility. Failure of PGCs to survive, proliferate or differentiate correctly in the embryo and fetus can result in sterility in the adult animal. In some cases normal PGC differentiation is perturbed and PGCs form pluripotent stem cells, termed Embryonal Carcinoma (EC) cells. These in turn form testicular tumors, teratomas or teratocarcinomas, which are the most common form of cancer in young men. Little is known about the molecular mechanisms responsible for guiding normal PGC differentiation and which are perturbed during testicular carcinogenesis. In vitro, PGCs cultured in the presence of Kit ligand (KL) and leukemia inhibitory factor (LIF) proliferate as long as they would in vivo. But when basic Fibroblast growth factor (bFGF or FGF2) is added, PGCs continue to proliferate and give rise to pluripotent stem cells termed Embryonic Germ (EG) cells. This process mimics the formation of testicular tumors in vivo in which PGCs give rise to pluripotent EC cells. Although EC and EG cells are pluripotent, our preliminary data show that PGCs cannot form any other cell type and are therefore considered nullipotent. Thus the conversion of a PGC into an EC or EG cell represents a conversion from a nullipotent to a pluripotent state. The ability to manipulate PGC potency in vitro with bFGF provides a unique system with which to study the control of developmental potency in mammals. We designed a novel retroviral gene transfer system to dissect and manipulate the signaling pathways activated in PGCs by KL, LIF and bFGF to determine the relative importance of these pathways in stem cell development. Understanding the molecular mechanisms controlling the conversion of a PGC into a pluripotent stem cell will fill gaps in our knowledge of developmental potency regulation, testicular tumor formation and germline development as well as generating general information about stem cell physiology. The Specific Aims of the proposal are: i) To determine the role of bFGF in altering developmental potential of PGCs, ii) To define the mode of action of bFGF in stem cell development, iii) To define the role of bFGF in pluripotent stem cell formation in vivo and iv) To identify genes up-regulated in PGCs following exposure to bFGF.